STORY ARCHIVE

Midget Subs

66 years ago, almost to the day, one of the world’s most technologically advanced submarines of its time disappeared after an audacious raid on Sydney Harbour.

On 31 May 1942, three Japanese submarines stole into Sydney Harbour and brought the Pacific war to the suburbs of Australia’s largest city. These daring invaders were no ordinary submarines. They were fast electric midget subs, secret weapons that launched the attack on Pearl Harbour six months previously. Carried thousands of kilometres on the decks of giant mother submarines, each was released with their two-man crews and two torpedoes outside Sydney Heads. Their mission was to sink any aircraft carriers and battleships they could find at anchor in the unsuspecting harbour, and then return to the mother subs.

One became entangled in a net, and was blown up. A second was depth–charged to the bottom. The third sub, the M24, managed to fire its torpedoes, sink a ferry, kill 21 sailors, and escape in the early hours of the morning.

But the fate of the M24 and its crew became an intriguing wartime mystery that lasted more than six decades. Until two years ago, when the wreck of the M24 was found by recreational divers a few kilometres offshore from Sydney’s northern beaches. To plan the security of the rare wreck site – and what to do with more than fifty kilos of unexploded TNT on board - maritime archaeologist Tim Smith is launching the first scientific investigation of the M24’s last resting place.

Mark Horstman follows a story of retro technology and sheer guts to take us back to a Sunday night in 1942, when submarines took Sydney by surprise.

TRANSCRIPT

Mark Horstman: We’re heading out to the wreck site of M24, M for midget, the Japanese submarine that attacked Sydney Harbour.

Narration: It doesn’t get much better than this for NSW’s senior maritime archaeologist, Tim Smith.

Tim Smith: This is one of the most unique shipwrecks in Australia, but importantly it’s one of only five detected in the entire world, so archaeologically it’s incredibly rare.

Narration: These midget submarines were absolute cutting edge, there was nothing like them in the world at that time.

But for the remains of two sailors inside, the M24 is also their tomb.

Mark: How the sub took Sydney by surprise is a story of retro technology and sheer guts, that takes us back to a Sunday in 1942...

Night is falling, as a southerly wind builds a rising swell…

Newsreel: Sydney Harbour…naval depot and vital shipping centre…chosen as the scene of Japan’s daring assault on the night of May 31…

Narration: Five of the world’s biggest diesel electric submarines, the 2500-ton Japanese I-class, lurk just off the coast. Each is so massive it carries a hundred men.

Over five hundred Japanese Imperial Navy crew on the doorstep of Australia’s largest capital city.

On board is their secret weapon: a 46-ton electric midget sub, which despite its name, is 24 metres long.

They were very fast. In fact, they were one of the fastest submarines in World War II. They could do 20 knots at a sprint, but only for fifteen minutes.

Each midget piggybacks on the rear deck of its mother sub. Once the batteries are fully charged, it’s released underwater.

On this night, three mini-subs are deployed. Each carries two elite commandos and two torpedoes. Under cover of darkness, they slip into the unsuspecting harbour. Their orders are clear: sink the biggest ship, then return.

Japanese Comander 1942: If there is a battleship or aircraft carrier before the Harbour Bridge, then attack it.

With only a periscope, they find their target: the warship USS Chicago.

Ashibe has to crawl forward to arm the torpedoes. Ban has to line up the target under intense gunfire.

Tim Smith: At the same time because no one was steering the submarine, the commander of the submarine would have his foot on the steering wheel here whilst navigating the submarine through the periscope.

Narration: From an almost impossible angle, they fire. Both torpedoes narrowly miss the warship, but one destroys the ferry Kuttabul, tragically killing 21 sailors.

The other two subs never get to do the same. One is blown up when it becomes entangled in the net. The other is bludgeoned by depth charges.

The two wrecks – and their dead crews - are dragged out of the harbour the next day. The subs are put on show as trophies of war.

Meanwhile, engineers pull the subs apart. The technology of the secret weapon is secret no longer.

Tim Smith: These plans are so revealing because they really tell you immediately how confined these craft were. If you’re looking at the plans…

Mark: Wow.

Tim Smith: They’re nearly all battery. So it’s the first thing that jumps out to me is the …Two hundred and eight batteries in racks of four. And they were either side of the conning tower.

Narration: More than 200 car batteries of two volts each drive a 600 horsepower engine, as powerful as a tank. Used carefully, they could last up to 12 hours.

Mark: The two-person crew had to fit just in this small section. The rest was either torpedoes or batteries.

Unlike the crews, the midgets were on a one-way trip. It wasn’t possible to reload them back on board the mother subs. The plan was to scuttle them with demolition charges of 50 kilos of TNT.

Tim Smith: The submarine was made of three main sections bolted together. So to put the explosive charge at those junctions would’ve … Split it apart….led to the sinking of the sub.

Narration: But what happened to the third sub, the M24, remained a mystery. Ban and Ashibe managed to evade the harbour defences, including the metal detector at the entrance.

Mark: The first line of defence for Sydney Harbour against submarines was an underwater electromagnetic cable that ran from North Head, down here into South Head, and right across to Middle Head. Now it did detect a submarine leaving the harbour early in the morning after the attack. And that was the last trace of the M24.

Narration: Japanese records show it made no contact with the waiting mother subs.

It took until 2006, when a group of recreational divers stumbled upon the wreck of the M24, just north of Sydney Heads.

Tim Smith: The submarine may well have been damaged, difficult to manoeuvre. She may have been running out of battery power. It may be that the submarine became essentially a dead submarine and no power, and was forced north by the southerly stream in wind that was blowing that night.

Narration: We’re looking at the front of the submarine, the bow, and this is the upper torpedo tube. Clear evidence that this is the midget that got into the harbour, fired its two torpedoes, and got out.

Tim Smith: There’s no evidence on the structure that we can see that suggests that there was any catastrophic damage, so it seems likely that it was deliberately settled to the seafloor at this point.

Narration: Today Tim, with the help of the Manly Hydraulic Laboratory, is starting to measure corrosion rates to know how best to protect the wreck.

Tim Smith: Now one way we can do that is by putting cathodic protection on the side, placing anodes in contact with the hull which can corrode preferentially to the steel of the submarine itself.

Narration: Twice every second, their profiler measures what makes it rust - the seawater’s temperature, pH, dissolved oxygen, and conductivity as it sinks 54 metres to the sandy bottom.

It may be too expensive and damaging to raise the sub, but Tim wants to glean every scrap of information he can, like the composition of original air pockets.

If we can actually identify and extract some air samples, we can have those analysed and see the nature of the air, whether there are any contaminants, the level of CO2 and carbon monoxide.

Over the years, the wreck has snagged many trawl nets, which ripped away parts of the sub and may have rolled it over. Now it’s shrouded in a forest of ghost nets and fishing floats.

One of our critical needs is to actually have these nets cut away from the vessel because they act to destabilise the structure as they’re moved in swell and surge activity.

And what of the demolition charges on board? Six decades of being dragged by trawlers hasn’t set them off, but Tim can’t be sure that no risk remains.

Tim Smith: That’s one of our critical factors. We know the explosives are in steel canisters, which are air-tight, and if they have not corroded substantially, they could still perhaps be dry.

Narration: But more than a shipwreck, this is a war grave.The fact that the ladder is in stowed position and the hatch appears to be locked down, is quite clear evidence that the crew, Ban and Ashibe, never left the submarine.

Tim Smith: Sand and silt level is actually here. So, it’s about that far below the periscope. So all this is covered by sand and silt.

Narration: And where do you think the bodies of the submariners might lie?

The control room’s one likely space today that’s buried in sand up to about that level, so there’s quite a depth of sand deposit that could conceal any human remains in that passage.

The unexploded weapons and the human remains mean the site is protected by Federal law and high-tech alarms that detect the sound of any trespassers.

Tim Smith: It’s the only system of its kind in Australia that protects an historic shipwreck. The site’s protected by law but it is also protected day to day with active surveillance.

Narration: Back in 1942, the bravery of the submariners killed in Sydney Harbour so impressed the Australian Navy they were given a military funeral.

66 years later, the last resting place of the remarkable M24, and its commander and navigator, continue to be honoured as their crewmates were.